Deflection rollers are used in many printing and laminating machines. The journal ends of the shaft of the paired cylinders are mounted in journal bearings. The bearings on one or both of the cylinders are attached to a moveable arm to move the cylinders into and out of contact with each other and to set the pressure applied to the web at the nip point. The frictional driving force on the web produces an equal reaction force on the cylinders. Since the cylinder shafts are fixed at the ends, this reaction force produces a torque that increases proportional to the distance from the fixed end points, becoming greatest toward the middle and tending to causing a bend of the roller cylinders that can result in uneven nip pressure across the web. If no compensation is made to counter the roller bending, the web will be stretched at the edges more than in the center, creating a risk of tearing or wrinkling. The uneven pressure caused by the bending can also result in non-uniform transfer of ink or creases in the lamination.
There have been many techniques developed to compensate for the torque and reduce the bending. In heavier roller assemblies, mid-point or intermediate bearings for the shaft inside the roller cylinder may have an adjustable eccentric collar to produce a counterforce at the bearings, as described in U.S. Pat. Nos. 2,261,740 and 4,637,109. Another known technique is to pre-set a counter torque on the shaft by adjustable angle journal boxes for the shafts, for example by changing the angle of the bearings by way of adjustment screws in the bearing sleeve, as described in U.S. Pat. No. 5,052,294. The torque can also be applied by an eccentric bearing.
Another technique has been the use of crowned roller surfaces, where the elastic cover of the roller tapers from a higher crown in the center section to a reduced diameter toward the ends. The term “crown” is usually used to denote the shape or diameter profile of the roller necessary to compensate for deflection in order to maintain a uniform nip pressure distribution. Since roller deflection is dependant upon the roll dimensions, the elastic material, and the load applied, the crowning profile is generally matched to a particular roller configuration and constant operating loads. Common profiles can be roughly approximated using a 70 degree cosine curve to approximate the bending curve of a simple beam under uniform load. In heavier loads, rollers with long lengths may start with a longer profile up to a 90 degree cosine curve. Even after approximating the profile, however, the crown is usually adjusted by experimenting with nip impression paper to get the finished crown.
Since the crown profile is selected to optimize uniform pressure under a fixed set of conditions, changing the nip pressure for any reason is likely to reduce the uniformity. With relatively long slender rolls as often used in laminating machinery and some printers, it may be useful or necessary to increase the nip pressure depending upon the thickness of the laminating films or other web materials and the selected operating speed. Consequently, it would be useful to have a convenient method and versatile apparatus that can compensate for these pressure variations over a wide range of conditions.